Copper electrodeposition process and anode for use in same



U ed States Pat COPPER ELECTRODEPOSITION PROCESS AND ANODE FOR USE INSAME Gerald C. Van Tilburg," Port Chester, N.Y., assignor to AmericanMetal'Climax, Inc., New York, N.Y., a corporation of New York r.

No Drawing. Application'March 19, 1957 Serial N0- 646,985

4 Claims. (11. 204-52 formation and further resulting in the formationof an improved copper deposit on the cathode.

In copper electrodeposition processes commonly re- 1 ferred to as copperelectroplating, the problem of roughness of the. cathode electrodepositi.e. the copper deposit on the cathode is of extreme importance.Although the use of various addition reagents to the bath compositionhas proven helpful for improving the properties and characteristics ofthe copper deposit on the cathode, this approach has not proved entirelysatisfactory as a solutio of the problem. Y

-A major cause of rough cathodic deposits is considered to be theformation and presence of anode sludge which occurs during the course ofthe electrodeposition process. The formation of such sludge is thoughtto be due to a combination of factors including electrochemical actionat the face of the anode, and a preferential attack by the electrolytesolutions on the boundaries between the crystals of the copperconstituting the structure of the anode. Such attack has the result thatloose particles of the anode are detached from the body of the anode,particularly when the crystals are small, and are carried into theelectrolyte forming a sludge which settles mostly in the bottom of thetank. During the process, however, some of these loose particlesaremechanically carried to the cathode where they produce a deleteriousrough plate upon the cathode. Another effect of such intercrystallineattack is an uneven anode dissolution which results in excessive lossesof copper in terms of the anode remmant which must then beprematurelydiscarded.

Several methods have been suggested to overcome the undesirable effectsof such sludgingor to reduce the formation of anode sludges Oneproposalinvolves bagging of the anode whereby the sludge is caught in the bags.Others have proposed the addition of elements to copper anodes made ofordinary that is, oxygen bearing or tough pitch copper whereby chemicalfilms are formed on the surfaces of the anode to inhibit sludgeformation.-

certain advantages in some instances, such anodes have generally'beenfound to possess limited applicationsw'ith respect to the bathcompositions suitable for use therewith.

In order to obtain more uniform anode dissolution, there is evidence toindicate that dissolution should occur through the metal crystal ratherthanat the crystal boundary; For best results,'it becomesjaccording lyapparent that the consumable copperanode should possess a substantiallyuniformly dense crystal structure as well as clean crystal boundaries.The need for these requirements are obvious since non-uniform"density'of thefcopper would incur'more rapid dissolution through theless dense portion resulting in unevenoverall -dissolution,of the anode."Then too," the presence oroxygen "0'1'" oxides and other impurities intli'e'grain 'boundary'whichf are .usually more soluble than'copper leadto grain boundary corrosion which ultimately causes increasedsludgeflformation.

In view of the above, the improved platingjcharacteristics and moreefficient utilization of the anode made possible by the more recentlyincreasing use of'high purity, oxygen-free, high conductivity copperanodes in conventional acid type and alkaline cyanide electrodepositionprocesses are notsurpri'singa'since such anodes are characterized by (1)higher purity, (2) more uniform and increased de nsity and (3) theabsence in 'the copper crystal boundaries of oxygen or oxygen incombination witha chemical deoxidant'asis'normally found inanodes/composed of tough' pitch of chemically deoxidize'd copper. i n xIn accordance with the' foregoing, it will be seen that a single, pure,dense crystal of copper would be ideal for the purpose ofelectroplating. Since this is presently impossible commercially,a'feasible approach to the problem consists of making available a'copperanode'which possesses the combination of (1) largercrystal size,-(2-)uniform density of the crystals','and (3) minimum number of crystalboundaries which should be as clean as'pos- 'sible particularly of oxideinclusions. l

I havenow found that by the addition of extremely small quantities of:lithium to oxygen-free high conductivtiy copper, a substantial averageenlargement'of the crystal structure is obtained whereby it becomespossible to reduce in anodes 'of equivalent size the overallnumber andextent of. crystal boundaries presented to the sludge-formingpreferential attack of electrolyte solutions during. theelectrodeposition process; Moreover, I have found that none of the otherdesirable properties connected with the use of oxygen-free, highconductivity copper anodes in either an acid sulfate or fluoborate asWell as a cyanide copper electroplating process are in an way iadverselyaffected by such lithium additions;

It is the principal object of this invention to provide a copperelectrodeposition process utilizing an oxygen-free, high purity copperanode having an enlarged average crystal structure whereby preferentialattack during ele'ctroplating is minimized and the more uniformdissolution of the anode results in reduced sludge formation andimproved smoothness of thecopper deposit on the cathode. a

' Another object of thisinvention'is'to provide an'oxygen-free, highpurity'copper anode having a relatively enlarged crystal structure whichis conducive to improved results in copper electrodeposition processes.

Other specific objects and advantages of the invention will becomeapparent as this specification proceeds.

For the purpose of thepresent invention, it is essential that the copperused for the fabrication of the anodes be oxygen-free high conductivitycopper. By the term oxygen-free copper" is meant a high purity copperwhich contains no oxygen by .virtue of the process used in production inaccordancewith known methods therefor exclusive of the use of chemicaldeoxidizers such as phosphorus, and the like. Thus, copper which hasbeen produced in a reducing atmosphere such asthe commercially availableOFHC brand copper may be used as well as copper prepared in an inertatmosphere or in a vacu- 11m. Such copper is usually of 99.98% or evenhigher purity and theextremely small amount of impurities which 'im y bepresent is not objectionable for the stated purpose. Atypical analysisof the composition of OFHC brand copper with respect to the impuritycontent thereof is illustrated by the following:

. Percent Silver 0.001 Iron 0.001 .Sillflit' 0.002 TLead L 0.0003 Tin p0.0001 Nickel 0.0008 Antimony Less than 0.0005 Bismuth Less than 0.0001Zinc Less than 0.0003 Manganese Less than 0.00005 Oxygen None The copperanodes for use in the electrodeposition processes as herein contemplatedare made by the addition of extremely :small amounts of lithium to theoxygen- :free high conductivity copper using oxygen-excluding :practices.in accordance with known practices during the :entire process. Theamount of added lithium in the form of a master alloy or-as lithiummetal may be varied between 0.0001 to 0.001% by weight. Preferably,between 0.00015 to 0.0003% may be used with optimum results beingachieved with the use of about 0.0002%.

The lower limit of 0.0001% for the amount of lithium :to-be added to theanode metal'is determined by the observation that any'smaller additiondoes .not achieve the (desired crystal enlarging effect. While the upperlimit is :less critical, it is desirable to keep the addition to thenecessary minimum not only for cost considerations but also toensure ananode of maximum copper purity. For this reason the use ofabout 0.0002%as previously indicated is most desirable in that adequate crystalenlargementis achieved without any appreciable impairment of purity,conductivity or density of the oxygen-free high conductivity copper.

Although the reasons for the unexpected grain enlargement achieved inoxygen-free high conductivity copper by the addition of such extremelysmall amounts of lithium are not clearly understood, it is believed thatthe highly eflicient scavenging action of lithium serves in some man-.ner to effect a reduction in the nucleation centers present inthecopper. Theeffect is unique, however, since the .efliect associated withlithium addition to ordinary copper even in such small quantities isgenerally grain refinement as opposed to the actual grain enlargementobtained in the case of oxygen-free copper.

Lithium-treated oxygen-free copper anodes preferably made by theaddition of the desired amount of lithium .metal to a copper meltemploying conventional oxygenexcluding practices during the melting,lithium addition and casting steps may be used in the usual manner usingconventional bath compositions of the acid or alkaline type. By way ofspecific illustration, a conventional aqueous acid sulfate plating bathconsisting essentially ofcupric sulfate and sulfuric acid may be usedutilizing, for example, acurrentdensity ofabout 100 amperes per squarefoot of anode surface. Similarly the new and improved anode may beadvantageously used in conventional cyanide plating processes utilizingan electrolyte bath composition, for example, which consists essentiallyof an aqueous solution of copper cyanide and sodium and/ or potassiumhydroxide with anode current densities averaging about 15 amperes persquare foot.

Whether or not the advantageous results and improvements areattributable solely to the enlarged grain structure perse of themodified anodes herein described cannot be positively ascertained atthis time. Whatever the reasons may be for the superior performance ofthe lithium-treated oxygen-free copper anodes of the type hereindisclosed, the fact remains that, in addition to the adaptability of theanode to both acid and cyanide processes, smooth and fine-texturedcathode deposits of substantially pure copper are consistently obtainedwhich are substantially free from nodular growths notwithstanding thethickness of the deposited plate. Furthermore, the reduced sludgeformation and minimized cathoderoughness resulting therefrom entirelyeliminates the need for bags or diaphragms around the anodes. The moreuniform dissolution characteristics imparted to the anode in addition toreducing copper losses by sludge formation also permits a more completeutilization of the anode during the electrodeposition process wherebyscrap losses are appreciably less. I

Since certain changes maybe made in the above invention and differentembodiments of the invention may be made without departing from thescope hereof, it is intended that all matter contained in the disclosureshall be interpreted as illustrative and notin a limiting sense.

I claim: I

1. In a copper electrodeposition process wherein an electric current ispassedvfrom a consumable copper anode through an electrolyteto a cathodeto provide a deposit of substantially pure copper on said cathode, theimprovement which comprises subjecting to the action of said current ananode consisting of oxygen-free high conductivity copper to which isadded from '0.0.001'% to 0.001% by weight of lithium to effectdissolution of improved uniformity of said anode with minimal sludgeformation while forming a cathode deposit free of nodular growths.

2. In a copper electrodeposition process wherein an electric current ispassed from a consumable copper 'anode through an electrolyte to acathode whereby a deposit of substantially pure copper is effected onsaid cathode, the improvement which comprises subjecting to the actionof said current an anode consisting of oxygenfree high conductivitycopper containing theimpurities normally present therein to which isadded from 0.00015% to 0.0003% by weight of lithium toeifect moreuniform dissolution of said anode whereby less sludge is formedduring'the process.

3. The processofclaim 2 wherein theelectrolyte is an acid electrolytecomprising an aqueous solution of cupric sulfate and sulfuric acid. 7 V

4. The process of claim 2 wherein the electrolyte is otthe alkalinecyanide type.

References Cited-in the file of this patent UNITED STATES PATENTS.

1,812,992 Smith July '7, 1931 2,689,216 Nevers et a1. Sept. 14, 19542,690,997 Jernstedt Oct. 5, 1954 2,809,929 OStrow-et-al. Oct. 15, 1957

1. IN A COPPER ELECTRODEPOSITION PROCESS WHEREIN AN ELECTRIC CURRENT ISPASSED FROM A CONSUMABLE COPPER ANODE THROUGH AN ELECTROLYTE TO ACATHODE TO PROVIDE A DEPOSIT OF SUBSTANTIALLY PURE COPPER OFF SAIDCATHODE, THE IMPROVEMENT WHICH COMPRISES SUBJECTING TO THE ACTION OFSAID CURRENT AN ANODE CONSISTING OF OXYGEN-FREE HIGH CONDUCTIVITY COPPERTO WHICH IS ADDED FROM 0.0001% TO 0.001% BY WEIGHT OF LITHIUM TO EFFECTDISSOLUTION OF IMPROVED UNIFORMITY OF SAID ANODE WITH MINIMAL SLUDGEFORMATION WHILE FORMING A CATHODE DEPOSIT FREE OF NODULAR GROWTHS.